Field of Invention
This invention relates to the field of sensing flames in equipment such as gas furnaces using the electrical properties of flames. In such equipment it is necessary to sense (detect) that a flame is actually being produced when fuel is being provided to a combustion burner. Otherwise the unburnt fuel will continue to flow and build up, and may cause asphyxiation and if it finds an ignition source may explode.
The term “combustion” means the process of oxidation of molecules of combustible substances that occurs readily at high temperatures with the release of energy. It is accompanied by that phenomenon which is called “flame” and by the generation of “heat energy”.The term “flame” means a self-sustaining propagation of a localized combustion zone at subsonic velocities.The term “combustion burner” means a device used for facilitating the combustion of a gas or a liquid. The term “burner” means the same as combustion burner.The term “flame conductivity” means the electrical conductivity of a flame. The unit of conductivity is the “mho”. The term “flame conduction” means the same as flame conductivity.The term “flame resistance” is the reciprocal of flame conductivity. The unit of resistance is the Ohm.The term “flame rectification” means the property of flames to preferentially conduct electrical current depending on the direction of the electrical current.The term “flame electrode” means an electrically conducting material immersed in a flame (when a flame is present), and which is electrically isolated from the combustion burner (except for a flame) and which may be electrically connected to something outside of the flame. The term “flame probe” means the same as flame electrode. The term “flame rod” means the same as flame electrode. The term “flame sensor” means the same as flame electrode.The term “flame battery” means the voltage produced between a combustion burner and a flame electrode that is immersed in the flame produced by the combustion burner. The term “flame voltage” means the same as flame battery.The term “flame proof” means proof that a flame exists. The term “proof of flame” means the same as flame proof.The term “plasma” means a collection of gas where a large proportion of atoms have enough energy that their electrons have been stripped away, creating ions, and that the proportion of ions to intact atoms is high enough that Coulomb forces have a significant effect on the behavior of the collection of gas. The ions creating the plasma will be termed “plasma ions”.The term “chemical ions” means reactive molecules, or atoms, that have unpaired electrons. The term “chemi-ionization” means the process by which molecules, or atoms, come to have unpaired electrons. The terms “chemi-ions”, “radical”, and “free radical” mean the same as chemical ions.The term “thermionic emission” means the emission of electrons from the surface of an electrically conducting material when the material is heated to a temperature high enough to overcome the work function of the material, typically several electron volts. One electron volt is equal to approximately 1.602×10−19 Joules.The term “high impedance buffer” means a buffer whose input impedance is substantially higher than the impedance of the circuit it is intended to buffer. The terms “amplifier” and “buffer” will mean the same thing regardless of the gain of the circuit.The term “mixer” means a circuit that accepts two signal inputs and forms an output signal at the sum and difference frequencies of the two signals. The terms “mixing” and “to mix” mean using a mixer. When two signals are mixed in this manner it is also called heterodyning.The term “flame good indicator” will mean the same as “indicator”.The term “symmetrical square wave” means a square wave having a duty cycle of substantially 50%.
Prior Art
The electrical properties of flames comprise flame conduction, flame rectification, and the generation of a flame voltage between a metal burner and a flame rod.
U.S. Pat. No. 1,688,126 Method of and Apparatus for Control of Liquid Fuel Burners issued Oct. 16, 1928 to R. F. Metcalfe, assigned to Socony Burner Corporation {IDS Cite 1}. This patent teaches using the resistance of the flame for providing flame proof. It uses only the flame resistance, not flame rectification. Two electrodes are used (Contacts 7 and 8 in Metcalfe FIG. 1). From page 3, right column, lines 70-79:                One of the main features of my invention is to utilize the phenomenon of the variation in resistance to the passage of sparks between any two contacts. The resistance in this instance is offered by the gases within the combustion chamber 4. I have found that I may take advantage of this phenomenon by utilizing the resistance to the passage of sparks between the points of the spark-plug employed for igniting the combustible mixture.In Metcalfe FIG. 1 the secondary of Spark Coil 56 is used to produce an ignition spark between contacts 7 and 8. During ignition the resistance of the burning gas is reflected back through to the primary winding of Spark Coil 56. Since a spark coil has a high ratio of turns between the primary and secondary windings the resistance reflected back through the primary is much lower than if it was used directly. This lower resistance through the primary winding is apparently low enough to operate a relay (Electro-Magnet 58).It appears that the spark is continuously produced. Later patents note that the continuous spark causes radio interference and they teach systems that do not require a continuous spark.        
U.S. Pat. No. 2,112,736 Flame Detector issued Mar. 29, 1938 to William D. Cockrell, assigned to General Electric {IDS Cite 2}. This patent teaches using flame rectification for providing flame proof. Cockrell FIG. 1 shows an embodiment using one electrode (22) with the burner (2) used as the return. The AC used in the flame sensing circuit is used only for the flame sensing circuit and is not also used as a spark igniter. See Page 1, left column, line 41-Page 2, right column, line 15.
U.S. Pat. No. 2,136,256 Furnace Control System issued Nov. 8, 1938 to A. L Sweet, assigned to General Electric Company {IDS Cite 3}. This patent also teaches using flame rectification for providing flame proof and is an improvement on 2,112,736. Sweet introduces an additional electrode to allow the flame rectification circuit to operate reliably with an oil-fueled flame. See Page 1, left column, line 4-Page 2, left column line 2.
However, the wires from the two electrodes are surrounded by a shield. See Page 6, left column lines 36-55 and Sweet FIG. 4. Shielding the wires reduces the stray coupling from the mains power (60 Hz in the U.S.). It is possible that the problem Sweet has solved is the stray coupling from the mains power which may be made worse by the use of oil as a fuel.
U.S. Pat. No. 3,301,307 Device for detecting the configuration of a burning flame issued Jan. 31, 1967 to Kazuo Kobayashi, et al, assigned to Ngk Insulators Ltd {IDS Cite 4}. This patent teaches the use of the flame battery for flame proof. From Column 2, lines 3-15:                The principle of the invention is based on, first of all, the recognition of the phenomenon that a negative potential to ground is produced in an electric conductor when it is located in a burning flame. It seems that such a phenomenon is due to an exchange of electric charges between the conductor acting as an electrode and ionized molecules through the contact surface of said electrode with the flame depending upon differences of temperature and degree of combustion between the inner and outer parts of said burning flame and atmospheric conditions. The phenomenon is inherent to flames and a potential difference in the order of 2-10 volts or more has been obtained by experiments.        
U.S. Pat. No. 4,082,493 Gas Burner Control System issued Apr. 4, 1978 to Dahlgren, assigned to Cam-Stat, Incorporated {IDS Cite 5}. This patent also teaches the use of the flame battery for flame proof. See Dahlgren FIG. 2 and Column 3, lines 32-42.
U.S. Pat. No. 8,310,801 Flame sensing voltage dependent on application issued Nov. 13, 2012 to McDonald, et al., assigned to Honeywell {IDS Cite 6}. This patent teaches using flame rectification for providing flame proof. The claimed novelty is that in order to avoid excessive component stress, energy consumption, increased electrical noise, and contamination build-up, when accuracy is critical a higher voltage is used. Once a flame has been established, the AC voltage may be adjusted to a lower level. See Column 2, lines 10-44.
However, McDonald has not produced evidence that the use of a high AC voltage causes excessive build-up of contamination on a flame rod, increased energy consumption that generates extra heat, or that it stresses associated electronic circuitry. The commonly accepted theory is that contamination of the flame rod is caused by the products of combustion, notably carbon. Also, any extra heat that might be produced would not be wasted because the purpose of a furnace is usually to produce heat. It is likely that the real value of McDonald's system is that, since his high voltage AC is produced electronically, it is isolated from the AC mains. This is in contrast to the commonly used practice of using the un-isolated AC mains for the flame rod voltage. Since the combustion burner is typically used as the electrical return path for the flame rod and is electrically connected to the equipment cabinet (which is required to be grounded) this requires that mains neutral and mains ground be connected. According to the National Electrical Code this may only be done (and is required to be done) at the service entrance to the building and no place else. As a result, an electrical connection problem outside the furnace at the service entrance may cause a flame sensing circuit to malfunction even though there is no problem in the furnace itself. Since McDonald's invention produces the high voltage AC for the flame rod electronically (and is isolated from the mains) it would not be subject to this failure mode.